Process of producing stannic oxide



June 28, 1949. s. M. BAXTER PROCESS 0F PRODUCTNG STANNIC OXIDE Filed Nov. 27-, 1943 rush. 0.

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www5@ www QU Patented lJune 28,` 1949 UNITED STATES ATENT OFFICE PROCESS F PRODUCING STANNIC OXIDE Stanley M. Baxter, Burlington, Mass.

Application November 27, 1943, Serial No. 511,958

1 Claim. l

This invention relates to a new method of separating element-al tin from its ores, ore concentrates, tin-bearing scrap, alloys of tin or other partially or wholly processed product containing tin, but more particularly to the production of tin ingots from naturally occurring tin-bearing mineral aggregates.

In the process of smelting tin ores as currently practiced, that is, by the conversion of the tin and any other metallic sulphides which may be present tc an oxidized form of the metal by roasting in air and the chemical reduction of the tin to the metallic state in blast furnaces or reverberatory furnaces with carbon, many of the impurities occurring with the tin are reduced to the metallic state before the tin or simultaneously with it; viz., copper, iron, lead, bismuth, antimony, zinc, silver, etc. These, therefore, contaminate the tin as it is produced and impart to it unacceptable properties, Most especially iron when reduced with the tin forms a kind of alloy called hard head which is useless unless it is resmelted to eliminate the iron. These circumstances make it necessary to resmelt slags, and hard head formed, to dross and sweat the tin produced and to resmelt large quantities of residuum from such drossing and sweating. Sometimes these processes must be repeated, each repetition ioccasioning considerable additional expense and loss of tin in the process. The final product from this treatment made from ores accessible to United States smelters is called smelterV tin and is too impure to be satisfactory for most uses. At the present time it is being electrolyzed with difculty and at considerable expense to produce tin of the purity demanded by the market.

Thus the present procedure for the smelting of tin ores is very expensive, complicated and ine'icient. No satisfactory alternative to it has however until now been discovered.

I have discovered that tin can be separated from its ores simply, cheaply and efficiently as a pure tin oxide which can be readily recovered as stannic oxide (SnOz), easily handled, and inexpensively and efficiently reduced to pure metallic tin. The apparatus required for the process is standard for present day smelting practice.

It is generally believed that both oxides of tin, stannous oxide and stannic oxide (SnO and S1102) are not volatile under furnace conditions of temperature and pressure. The first-named is supposed to decompose according to the equation 2SnO=Sn+SnO2 when it is heated in an atmosphere with which it is in equilibrium. It-has been variously stated by different authorities that under atmospheric pressure this will occur from 385 C. to 956 C. It was rightly reasoned that if this be the case, volatilization as stannous oxide cannot take place above this range of temperature. Stannic oxide is generally supposed to decompose at 1165 C. Without vaporizing. The boiling point oi metallic tin is 2400" C. It was therefore, assumed that tin and its oxides are not volatile under furnace conditions.

I have found this general belief to be in error. Stannous oxide does not decompose due to an increase of temperature only and neither does stannic oxide. Stannous oxide sublimes below l C. and vaporiaes increasingly rapidly above 700 C. i

For instance at 927 C., 1200" K., the dissociation pressure of the oxygen of stannous oxide is 1.3 1015 atm. and of stannic oxide, for the equation SnO2=SnO+ 1/202, is 7.95 1015 atm. If the dissociation pressure of the oxygen of a liquid such as slag, or the partial pressure of the oxygen of an oxygen-containing gas such as the furnace atmosphere be equal to either of these pressures and either of them constitute the environment of tin or its oxides, the corresponding oxide will be in equilibrium with the environment with respect to oxygen Iand oxygen will pass from or to the elemental tin or its oxide to or from the environment until the oxide of tin characteristic of the oxygen concentration of the existing environment has been completely formed and equilibrium so established. At the indicated temperature, metallic tin will be produced if the oxygen pressure of the environment be below 1.3 10*15 atm. If it be at or above that pressure stannous oxide will be formed, more rapidly with increasing oxygen pressure until it is equal to '7.95 1015 atm. at which pressure stannic oxide will be formed and the speed with which SnO will be formed will increase with increasing oxygen pressure.

The oxide will Vaporize as oxide at any temperature until the partial pressure or osmotic pressure of the gaseous or dissolved oxide of tin in the environment be equal to the vapor pressure at the surface of the oxide of tin. This escaping tendency for metallic tin or stannic oxide is extremely small but it is distinctly manifest for stannous oxide at 700 C. and increases rapidly with temperature. It is greater the lower the concentration of stannous oxide in the environment. The speed of vaporization of stannous oxide may therefore be increased by increasing the temperature or decreasing the osmotic or partial pressure of the stannous oxide of the environment or both. This decrease is easily caused by reoxidizing the stannous oxide or by continuously refreshing the environment with new stannous-oxide-free material. Such a procedure causes the continuous vaporization or solution of the stannous oxide.

If stannous oxide be in an environment of slag only and is heated to a point at which the stannous oxide is volatile and the slag is molten, its solution in the slag will take place until the osmotic pressure of theI stannous oxide in the slag is equal to the solution pressure of the solid stannous oxide in the slag and the stannous oxide will thereafter bubble through the slag until the partial pressure of stannous oxide in the atmose phere is equal to its osmotic pressure in the slag and therefore to the vapor pressure of the stannous oxide, at which point equilibrium will be established.

If the partial pressure of oxygen in the furnaceatmosphere be in excess of the dissociation pressure `of the oxygen of stannic oxide for the temperature ofthe atmosphere, stannous oxide which has passed through the slag will be immediately oxidized to stannic oxide and the atmosphere will be immediately purified of stannous oxide. Being in a state of molecular division it will be carried out of the furnace into the i'lues by the furnace draft, there to coagulate andprecipitate as' stable stannic oxide or to be caught by any good fume collecting device.

Happily thel impurities of tin ores` either are not oxidized or are not volatilized under the same conditions as is tin. Those such as arsenic and sulphur are completely eliminated during the roasting of the ore. Those which might be vola tileas oxides above 760 C. are not oxidized within the spread of oxygen pressure necessary for the-formation of stannous oxide but are reduced to metal within this spread of oxygen pressure'. Iron exists as oxide. The following is a short list of the `dissociation pressure of some metallic oxides at 927 C. (1200 l).

Cu2O=2v 1il8 copper will be present as nonvolatile metal.

PbO=7 l012; lead will be present as nonvolatile metal.

ZnO=llG-24 zinc will be present as nonvolatile oxide.

FeO=1.6 l0*19; iron wiil be present asnonvolatile oxide.

There therefore no inconvenience' experi'- enced in the process deriving from the volatilization ofimpurities;

They stannous oxide is volatilized solely. It alone is precipitated from the gaseous state in the furnace atmosphere forming molecularly di vided, solid particles of stannic oxide which coagulate and precipitate after having passed out of the furnace in a suspensoid state in the furnace gases' and are recovered under thev action of fume collectors in the form of pure stannic oxide powder.

This powder can be briquetted and smelted, to form pure tin ingots of 99.8% tin content or better, with any solid or liquid carbonaceous binder which does not contain objectionable elements.

The most convenient device for the establishment and control of the concentration of oxygen in the surroundings of the chargeA is the formation of an oxidizing slag, which surrounds and covers the tin containingv particles, the oxidizing power of which can easily be controlled through the use of solid carbon mixed? with the slag; To this end the gangue material" of the ore is uxed to produce a high iron subsilicate slag which melts at a temperature of about 1000 C. If the atmosphere of the furnace is oxidizing a part of the iron of the slag is oxidized to form ferric oxide in sufficient quantity to produce an oxygen dissociation pressure in the slag quantitatively more than the 7.95 10-15 atm. required to form stable stannic oxide from the tin of the charge. if solid carbon be added to and mixed with the slag in sufficient quantity the dissociation pressure of the oxygen of the slag will be reduced below the 1.3 10-15 atm. pressure required for the formation Aof stable stannous oxide and the tin will be reduced to' the: metallic state. If the carbon added be between these two limits the slag will oxidizel metallic tin or reduce stannic oxide to stannous' oxide. This volatilizes rapidly at the temperature of the molten slag, bubbles through the sla-g, is oxidized instantaneously by the furnace atmosphere and is carried out of the furnace into the ilues by the furnace draft. as finely divided, pure, stannic oxide.

Obviously anymetal' which forms a higher and a lower" oxide the oxygen dissociation pressure of the lower oxide of which is lower than the oxygen dissociation pressure of stannous oxide and whose oxygen dissociation pressure for the higher oxide is above the upper limit for the dissociation pressureV of the oxygen of stannous oxide may be used oxidizingragent in this slag.

It was found that this lcontrol of the oxidizing powerof the slag isA easy todo, the spread of the proportion of solid carbon, even powdered charcoal, required to maintain the system between the indicated required limits being considerable and requiring more.y than an hour to be oxidized away from the upper to the lower limit by the slag. f

Beside; the liquid. environment just described the tin. of the charge may be converted to stannous oxide in a gaseous environment. It requires only the adjustment. of the partial` pressure of the oxygen. of the gasto a span between the two limits indicated previously. This may he done by adjusting; the degree of oxidation of la flame in a furnace. by varying the air used for combustion or it maybe done by using a predetermined mixture of carbon monoxide: and carbon dioxide. The oxygen dissociation pressure of the former when pure is lower than that required for the stability of stannous oxide and that for pure carbon` dioxide is greater at any temperature than that required for the stability of stannic oxide. Betweenthese two limits there isa spread of 'partial pressure for carbon dioxide and its resultant oxygen dissociation pressure which varies with the: exposure,l of the charge to the atmosphere, that. is the degree of rlneness of the charge and the. degree of penetration of the charge `by the atmosphere, which will' cause stannous oxide to be .formed froml the tin of the charge.V

In this case the stannousA oxide isnot oxidized by the furnace gas: and other means must be provided for' oxidizing or otherwise changing its chemicalcharacter before any method of fume recovery can be successfully applied to it. By passing.` this. gas into ay dust-collecting device and the pure. gas-'mixture into a combustion chamber pureA stannicl oxide may be obtained by fume recovery apparatus,

Many other methods for controlling thev partial pressure of the oxygen indifferent gaseous mix-tures may be workedout varying in detail but they will beA basically identical with that just described.

The advantages of the invention as applied to the recovery of tin from its ores will be apparent from the following specific example, but it is to be understood that it is merely an example of the broad invention set forth herein. The processes embodying the invention as applied to the recovery of tin may be carried out with apparatus of standard character such as is well-known and familiar to those skilled in the metallurgical arts, any alterations or rearrangements required by the invention being obvious from the principles herein set forth.

The drawing is, therefore, entirely of a diagrammatic character and partially in the nature of a flow sheet and illustrates an apparatus for carrying out the process of the invention in the treatment of tin concentrate with liquid slag as the medium of oxygen control.

In the example illustrated in the drawing, the reverberatory furnace, a` Cottrell precipitator and a bag house are employed. These are well known in the art.

The tin concentrate, usually roasted, is mixed with solid carbon such as powdered anthracite coal, charcoal or lcoke and the usual fluxes in proportion required more than to reduce all of the tin to metal if the carbon be oxidized by the tin to carbon monoxide. In the :first reverberatory furnace illustrated, the tin is rst reduced to metal and the charge melted. Then the excess carbon is oxidized by the furnace atmosphere until white fume appears in the exiting gases. Thereafter only sufficient carbon is added to the charge so that a maximum of visible white fume continues to be produced in the exiting gases. These gases are first passed through the Cottrell apparatus which acts to precipitate a portion of the pure stannic oxide of the fume. Then the gases pass from the Cottrell apparatus to the bag house by means of which a further separation of the pure stannic oxide is effected in the usual manner. The pure stannic oxide thus collected from the Cottrell apparatus and the bag house is then briquetted in the usual manner with a. suitable reducing agent and smelted according to standard practice producing tin bars of better than 99.8% tin.

Before recharging the first or volatilizing reverberatory furnace about half of the molten slag is tapped from the end run hot into the second reverberatory type of furnace which acts to clean and reduce the slag and in this furnace the tin content of the slag is reduced to metal and hard head by continued heating but with an excess of carbon. When this tin product has accumulated suiciently in this reducing furnace it is tapped into a ladle and poured back into the first or volatilizing furnace. The clean slag from the reducing furnace is tapped and sent to waste. The gases from this reducing furnace are sent to the stack or may, if desirable, be treated in a second bag house.

The metals, which are more easily reduced to the metallic state than tin is, accumulate in the volatilizing furnace. They will be alloyed with tin in inverse proportion to the completeness to which the charge has been volatilized. They should be tapped at such a time thatl volatilization of the charge on the hearth is practically complete. The quantity of this alloy will be quite small. It may be used for the produce of solder bar or it may be purified of tin by continued treatment similar to that described as taking place in the volatilizing furnace.

In employing the invention as applied to the recovery of tin I have produced efficiently stannic oxide of nearly purity with only slight traces of lead and antimony from 28.5% Bolivian concentrate. From this product pure metallic tin can easily and cheaply be produced using only standard methods.

Having thus descri-bed the invention, what is claimed as new, and desired to be secured by Letters Patent, is:

The process of producing stannic oxide which consists in mixing solid carbon with a high iron subsilicate tin concentrate slag having a melting point of approximately 1000 C., heating said mixture to its melting point in a furnace having an oxidizing atmosphere, maintaining the proportion of carbon in the mixture to cause a dissociation pressure of the oxygen of the slag ibetween approximately 1.3 1015 and 7.95 1015, thus oxidizing the resultant metallic tin and reducing the stannic oxide to stannous oxide which being volatilized at the slag temperature bubbles through the slag and is instantaneously oxidized into stannic oxide by the atmosphere of the furnace, and recovering the stannic oxide so produced.

STANLEY M. BAXTER.

REFERENCES CITED The following references are of record in the 

